Defining Relative clauses ...
... are used to give information about a person, place or thing. The information is necessary - without this information we don't know what you are talking about!
I hate the man who is talking to Sarah.
Relative Pronouns (and related words)
For things we say which and for people we say who. For places, we say where and for possessions (e.g. my car, his brother) we say whose + noun.
I like the dress which your sister is wearing.
I like the man who your sister is talking to.
I like the shop where you bought that dress.
I like the woman whose brother I met.
- Often, we say 'that' instead of which or who.
- You don't need to say 'who' or 'which' if there is a noun after it.
I like the dress that your sister is wearing.
That's the man who I love = That's the man I love.
That's the man who loves me - 'who' is necessary here.
From: The Tiny TEFL Teacher